1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76

/// A convenience struct for applying implementing `TypeEq` trait. Since the trait
/// connects two types, this allows neither to be Self, and prevents issues with
/// asymmetry.
pub struct Constrain;

/// There are no situations where you should implement this trait except for the one
/// impl for `Constrain`, which connects all 
pub unsafe trait TypeEq<A, B> {
    fn as_eq(&self, a: A) -> B;
}

unsafe impl<A> TypeEq<A, A> for Constrain {
    fn as_eq(&self, a: A) -> A { a }
}

/// Type-level properties
pub trait Has<Prop> {
    type Val;
    fn get<P>(&self) -> &Self::Val
    where
        Constrain: TypeEq<Prop, P>;
}

#[cfg(test)]
mod tests {
    use ::*;

    #[test]
    fn test_type_level_accessors() {
        struct X;
        struct Y;

        struct Point<T> { x: T, y: T }

        impl<T: Copy> Has<X> for Point<T> {
            type Val = T;
            fn get<P>(&self) -> &Self::Val
            where
                Constrain: TypeEq<X, P> 
            {
                &self.x
            }
        }

        impl<T: Copy> Has<Y> for Point<T> {
            type Val = T;
            fn get<P>(&self) -> &Self::Val
            where
                Constrain: TypeEq<Y, P> 
            {
                &self.y
            }
        }

        let p = Point { x: 3, y: 5 };
        assert_eq!(&3, p.get::<X>());
        assert_eq!(&5, p.get::<Y>());
    }

    #[test]
    fn test_as_eq() {
        struct Pair<A, B> {
            a: A,
            b: B,
        }
        impl<A: Copy, B: Copy> Pair<A, B> {
            // silly example: could just have implemented for Pair<A, A>
            fn to_vec(&self) -> Vec<A> where Constrain: TypeEq<B, A> {
                vec![self.a, Constrain.as_eq(self.b)]
            }
        }
        let p = Pair { a: 1, b: 2 };
        let v = p.to_vec();
        assert_eq!(vec![1, 2], v);
    }
}